To change between miles and kilometres and vice versa, navigate to the following on your iPhone:

Settings –> General –> International –> Region Format

Then choose…

–> Canada for kilometres

or

–> United States for miles

Once that is done, just switch back to Google Maps and you should see the changes immediately. If you had already started a Google Navigation Beta route, you will need to end it and start again to see the changes. Check them out below: